1. Field of the Invention
The present invention relates generally to a liquid crystal display device, and more particularly to a liquid crystal display device which adopts a multi-domain vertical alignment mode.
2. Description of the Related Art
A liquid crystal display device has various features such as small thickness, light weight and low power consumption, and is applied to various uses, e.g. OA equipment, information terminals, timepieces, and TVs. In particular, a liquid crystal display device comprising thin-film transistors (TFTs) including thin-film transistors (hereinafter “TFTs”) has high image quality performance and, therefore, it is widely used as a monitor of a portable TV, a computer, etc., which displays a great deal of information.
In recent years, with an increase in information amount, there has been an increasing demand for higher definition of images and for a higher display speed. The higher definition of images can be realized, for example, by making finer the array structure which is composed of the above-described TFTs. Of display modes of liquid crystal display devices, a vertical aligned nematic (VAN) mode has a higher response speed than a conventional twisted nematic (TN) mode. An additional feature of the VAN mode is that a rubbing process, which may lead to a defect such as an electrostatic breakage, can be made needless by vertical alignment.
In particular, a multi-domain VAN (MVA) is widely put to practical use since the viewing angle can relatively easily be increased.
In the MVA mode, liquid crystal molecules are aligned in a plurality of directions when a voltage is applied to the liquid crystal layer, for example, by means of mask rubbing, devices of pixel electrode structures, or protrusions provided in pixels. Thereby, a multi-domain is formed, and an improvement in symmetry of viewing angle characteristics and suppression of an inversion phenomenon are realized. In addition, the viewing angle dependency of the retardation of the liquid crystal layer in the state in which the liquid crystal molecules are vertically aligned, that is, in the black display mode, is compensated by using a negative retardation plate. Thereby, the viewing angle dependency of the contrast ratio (CR) is improved. Furthermore, by applying a biaxial retardation plate with an in-plane retardation to the negative retardation plate, the viewing angle dependency of the polarizer plate is also compensated, and still higher CR viewing angle characteristics are realized.
For example, patent document 1 (Jpn. Pat. Appln. KOKAI Publication No. H11-258606) discloses a technique relating to the MVA. Patent document 1 discloses domain restriction means, such as a protrusion, a recess or a slit, as means for forming a multi-domain. In a concrete example, a protrusion is provided on a counter-electrode, and neighboring pixel electrodes on an array substrate are disposed at a predetermined interval or more, thereby realizing a multi-domain. Patent document 1 describes that in the prior art the protrusion needs to have a predetermined width or more and a predetermined height or more from the standpoint of alignment stability, and that if sufficient width and height are not obtained, the alignment stability would deteriorate, leading to image quality degradation such as roughness and lowering in response speed.
On the other hand, since the protrusion is provided within the pixel, there occurs local light leak due to a stepped portion of the protrusion or a decrease in transmittance due to a voltage drop. Thus, the fact is that the protrusion itself is a factor of degradation in image quality. Moreover, if the interval of pixel electrodes is to be increased, the opening ratio decreases and the transmittance lowers.
As has been described above, from the standpoint of the transmittance and contrast, it is desirable that the size of the protrusion and the interval of the pixel electrodes be as small as possible. In short, the size of the protrusion and the interval of pixel electrodes are determined from the relationship of trade-off between the optical characteristics and the alignment stability.
As other techniques for forming the multi-domain, there are known techniques disclosed in patent documents 2 to 4 (Jpn. Pat. Appln. KOKAI Publication No. 2004-205903, Jpn. Pat. Appln. KOKAI Publication No. 2004-341487 and Jpn. Pat. Appln. KOKAI Publication No. 2007-041126).
In recent years, also in the field of displays for mobile terminals, microfabrication has been progressed to realize very high definition of 300 ppi or more, and there is a strong demand for the capability of displaying motion video. Although the optical characteristics, such as transmittance and contrast, are in a sufficiently tolerable level, the response time is not sufficient and there is a demand for improvement in display quality at the time of motion video display.